Emission spectroscopy and mass spectrometry based on plasma sources is a well accepted approach to elemental analysis. It is desired that an electrical plasma suitable as an emission source for atomic spectroscopy of a sample should satisfy a number of criteria. The plasma should produce desolvation, volatilization, atomization and excitation of the sample. However the introduction of the sample to the plasma should not destabilize the plasma or cause it to extinguish.
One known and accepted plasma source for emission spectroscopy is a radio frequency (RF) inductively coupled plasma (ICP) source, typically operating at either 27 MHz or 40 MHz. In general, with an RF ICP source the plasma is confined to a cylindrical region, with a somewhat cooler central core. Such a plasma is referred to as a “toroidal” plasma. To perform spectroscopy of a sample with an RF ICP source, a sample in the form of an aerosol laden gas stream may be directed coaxially into this central core of the toroidal plasma.
Although such plasma sources are known and work well, they generally require the use of argon as the plasma gas. However, argon can be somewhat expensive and is not obtainable easily, or at all, in some countries.
Accordingly, there has been ongoing interest for many years in a plasma source supported by microwave power (for example at 2.45 GHz where inexpensive magnetrons are available).
However, at least until recently, atomic emission spectroscopy (AES) systems based on microwave plasma sources (also referred to as MP AES systems) have generally shown significantly worse detection limits than systems which employ an ICP source, and have often been far more demanding in their sample introduction requirements.
For optimum analytical performance of the emission spectroscopy system, it is thought that the plasma should be confined to a toroidal region, mimicking the plasma generated by an RF ICP source.
It has turned out to be much more difficult to produce such a toroidal plasma using microwave excitation than it is in the case of an RF ICP source. With an RF ICP source, a current-carrying coil, wound along the long axis of a plasma torch, is used to power the plasma. The coil produces a magnetic field which is approximately axially oriented with respect to the long axis of the plasma torch, and this, in turn, induces circulating currents in the plasma, and these currents are symmetrical about the long axis of the plasma torch. Thus, the electromagnetic field distribution in the vicinity of the plasma torch has inherent circular symmetry about the long axis of the plasma torch. So it is comparatively easy to produce a toroidal plasma with an RF ICP source.
However, the electromagnetic waveguides used to deliver power to microwave plasmas do not have this type of circular symmetry, and so it is much more difficult to generate toroidal microwave plasmas.
U.S. patent application Ser. Nos. 13/838,474 and 13/839,028 disclose some electromagnetic waveguide-based apparatuses for exciting and sustaining a plasma and which may produce a toroidal or quasi-toroidal plasma using a microwave plasma (MP) torch and a suitability configured waveguide cavity.
MP torches are usually made of fused silica or alumina and consist of three coaxial gas tubes, including an injector, and intermediate and outer tubes. While the outer tube provides the plasma gas, the analyte is carried into the plasma through the injector tube. As described in U.S. patent application Ser. Nos. 13/838,474 and 13/839,028, for example, the MP torch is positioned horizontally with respect to the width of a rectangular electromagnetic waveguide through two holes provided in the sidewalls of the electromagnetic waveguide which define the interior region or cavity of the electromagnetic waveguide. The electromagnetic waveguide cavity provides the required electromagnetic field to initiate and sustain plasma inside the MP torch.
Unlike the ICP torch, the length of which is not limited by any structural parameters, the length of the main plasma chamber of the MP torch must be equal to or longer than the width of the electromagnetic waveguide so that the plasma is confined inside the torch. In addition, the microwave power will generally be distributed across the width of the cavity or interior of the waveguide, so even if an MP torch which is shorter than the width of the electromagnetic waveguide was practically possible, only a portion of the microwave power in the electromagnetic waveguide would be coupled to the plasma. On the other hand, since the electromagnetic field intensity is negligible outside the electromagnetic waveguide, from a power coupling standpoint there is no point in elongating the torch to extend beyond the waveguide. Therefore, other considerations aside, it would appear that the torch and cavity are best suited to one another when the main plasma chamber of the MP torch is as long as the width of the electromagnetic waveguide cavity.
There is therefore a desire to provide an improved microwave plasma source which can provide improved performance which approaches that of RF ICP, together with characteristics such as small size, simplicity and relatively low operating costs.